Measuring the concentration of a target gas within the atmosphere at long ranges requires a highly precise, quantitative spectrometer. A low noise detector and method are also needed in order to detect extremely low concentrations. The problem becomes even more challenging when coupled with the effects of atmospheric scintillation and the complicated absorption spectrum of the atmosphere.
One approach to quantitative measurements of trace gases in the atmosphere is pulsed differential absorption lidar (DIAL). In this method, a laser source with a narrow linewidth first produces a pulse tuned on-resonance with a particular gas absorption, then a second pulse tuned slightly off-resonance, and the atmospheric transmission of the two successive pulses is compared. Neglecting the effect of the change in atmospheric absorption over the change in frequency, the ratio of the two pulse intensities corresponds to absorption by the trace gas of interest, because the static atmospheric absorption drops out of the ratio. Beer's law then allows the calculation of a concentration between the source and detector.
Unfortunately, this simple picture is complicated by the technical requirements placed on the laser transmitter (i.e., source). Nominally, the laser must have sufficient power to overcome the average atmospheric absorption in the operating wavelength range (about 0.2-2 dB/km, typically), be as narrow-band as possible to maximize sensitivity to the trace gas, have a stable frequency to reduce the effects of atmospheric slope, and have a high repetition rate providing the benefits of averaging. These are aggressive requirements to meet in a single laser system.
Therefore, there is a need for a gas detector that reduces or eliminates the above mentioned transmitter requirements.